1. Field of the Invention
The present invention relates to a semiconductor device having a circuit structured with thin film transistors (hereinafter, referred to as TFTs) and to a method for manufacturing the same. Particularly, the invention relates to an electro-optical device represented by the liquid crystal display panel and an electronic appliance mounted with such an electro-optical device as a part. Incidentally, in this description, the semiconductor device refers to the devices in general capable of functioning by the utilization of a semiconductor characteristic, i.e. an electro-optical device, a semiconductor circuit and an electronic appliance, in any, are fallen under a semiconductor device.
2. Description of the Prior Art
Recently, development has been proceeded for semiconductor devices having TFTs structured by using a thin film (thickness: approximately several to several hundred nm) formed on a substrate having an insulating surface to have a large-area integrated circuit forming TFTs. There is known an active-matrix liquid crystal display device as a representative example. In particular, the TFT using a crystalline silicon film in its active region is allowed to form various functional circuits because of its high electric-field mobility.
For example, the active-matrix liquid crystal display is formed for each functional block, with a pixel circuit for image display, a shift register circuit based on a CMOS circuit, such driver circuits for controlling the pixel circuit as a level shifter circuit, a buffer circuit and a sampling circuit, on one substrate.
The TFT has, at least, a semiconductor film, an insulating film such as a silicon oxide film or a silicon oxide nitride film, an interconnections of various metal materials or the like, and a pixel electrode. The interconnections include a source interconnection and a gate interconnection (including a gate electrode). The source interconnection and the source electrode connected to the source region, in many cases, are connected through the other interconnection.
Meanwhile, of among the active-matrix liquid crystal displays, the liquid crystal projectors using small-sized liquid crystal panels have being spread at a rapid pace to broaden the field of use. Due to this, there are demands for convenience. Development has being continued in order to advance size reduction, brightness increase, definition enhancement and price reduction.
The active-matrix liquid crystal display, used in a liquid crystal projector or electronic apparatus display, has a pixel region structured with several million pixels. Each pixel is formed with a TFT, and the TFT of each pixel has a pixel electrode. A counter electrode is provided on the counter substrate sandwiching a liquid crystal, to form a kind of capacitor having a liquid crystal as a dielectric. The potential to be applied to each pixel is controlled under TFT switching function to control the charge to the capacitor. Thus, the liquid crystal is driven to control the amount of transmission light thereby displaying images.
Because the capacitor gradually decreases in its capacitance due to current leakage, it forms a cause to vary the amount of light transmission and hence decrease the contrast in image display. For this reason, in the conventional, there has been provided a capacitance interconnection to provide, in parallel, a capacitor (holding capacitance) separately from the capacitor having a liquid crystal as a dielectric. The holding capacitance serves to supplement a capacitance lost by the capacitor having a liquid crystal as a dielectric.
However, when trying to form, in the pixel region, a holding capacitance using a capacitance interconnection and secure a sufficient capacitance, the opening ratio must be sacrificed. Particularly, it is to be fully expected that pixel size reduction be continued for the small-sized high-definition liquid crystal display as used in the liquid crystal projector, as long as definition enhancement is sought together with size reduction. For example, in order to realize display with definition as high as XGA (1024×768 pixels) on a liquid crystal display of a diagonal 0.7-inch type, each pixel has an area of as small as 14 μm×14 μm. Meanwhile, even in the case of providing a contact hole with an area of 1-μm square, the contact must be secured with an area of 3-μm square by extending one side at least by 1-μm each if considering a problem with coverage or the like. In the case of 14 μm on one side of one pixel, when a 3-μm-square contact is formed one, the opening ratio is lowered at least 4.6%. The number of contacts is of a significantly important problem amid the continuing pixel size reduction.
At present, brightness increase is coped with by increasing the opening ratio while definition enhancement is by increasing the number of pixels. However, amid continuing pixel size reduction, there is an extremely difficult problem in simultaneously satisfying the improvement in opening ratio and the increase in the number of pixels while designing a pixel structure with a sufficient capacitance secured. When trying to realize such a pixel structure, the number of processes naturally increases to make the process complicate. This results in a problem of worsened yield and semiconductor-device manufacture cost increase.
Meanwhile, there are cases that light leak current increases due to the light coming at a surface of a substrate not formed with TFTs of a light-transmission liquid crystal display or the light incident upon a top surface and irregularly reflected in the substrate, thus increasing an off-current (drain current flowing in a TFT off-state). The increase in leak current requires an increase of holding capacitance for compensation, causing a problem of lowering in opening ratio in the pixel region.
The present invention is an art for resolving the foregoing problem, and it is a problem to realize, by a reduced number of processes as compared to the conventional in respect of the structure of TFT and holding capacitance, a reliable active-matrix liquid crystal display device having a high opening ratio to make a display with definition. Meanwhile, it is a problem to realize high-definition image display even in a liquid crystal display device designed with a pixel size as small as ten and several μm square and in an electronic apparatus using, in a display part, such liquid crystal display device.